Another pair of words that I frequently see misused is principle and principal…

Principle – a belief or moral that governs behaviour and decisions; a fundamental truth
On principle, John refused to watch the parade.

Principal – first or leading rank (e.g. a School Principal); initial sum of money before interest and fees
As Principal of the local school, Mary was well respected by the parents.

The most common uses of these words are often confused and used incorrectly. The easiest way to remember which is which is to think of your pal being a principal.